Good content helps increase your PR as well. DoFollow backlink is also good for higher PR if they're coming from high PR sites.
Only back links? What is the criteria quantity or quality? If your answer is quality then quantity please consider the following scenario. Let say google decided to give page rank from scratch. Everyone will start with a zero page rank. Now what google will consider ? Quantity or or Quality? What is the definition of quality now as every one has zero page rank?
Quality would be from well established websites with a lot of great content. Also, In this case quantity would play a bigger role as well. Though I guess Google will never do such a thing.
I know google will probably will not do such a thing. My point was about the basic rules of page rank. What they really are?
Well - on PR and Blogs - aren't you leaving a backlink. Anyway, I think that niche forums could be best since you are actually promoting your site w/ a sig backlink.